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(#) Unnecessary lambda with `trim()`

!!! Tip: Unnecessary lambda with `trim()`
   Advice from this check is just a hint; it's "weak" warning.

Id
:   `TrimLambda`
Summary
:   Unnecessary lambda with `trim()`
Severity
:   Hint
Category
:   Correctness
Platform
:   Any
Vendor
:   Android Open Source Project
Feedback
:   https://issuetracker.google.com/issues/new?component=192708
Affects
:   Kotlin and Java files
Editing
:   This check runs on the fly in the IDE editor
Implementation
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/TrimDetector.kt)
Tests
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/TrimDetectorTest.kt)
Copyright Year
:   2025

The Kotlin standard library `trim()` call takes an optional lambda to
specify which characters are considered whitespace.

When converting Java code to Kotlin code, the converter will convert
calls for Java's `s.trim()` into `s.trim() { it <= ' ' }`. This
preserves the exact semantics of the Java code, but is likely not what
you want: the default in Kotlin uses a better definition of what
constitutes a whitespace character (`Char::isWhitespace`) and also
results in less bytecode at the call-site.

!!! Tip
   This lint check has an associated quickfix available in the IDE.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/test.kt:3:Hint: The lambda argument ({ it <= ' ' }) is unnecessary
[TrimLambda]
    s.trim() { it &lt;= ' ' } // HINT 1
             -------------
src/test.kt:4:Hint: The lambda argument ({ it <= ' ' }) is unnecessary
[TrimLambda]
    s.trim { it &lt;= ' ' } // HINT 2
           -------------
src/test.kt:5:Hint: The lambda argument ({ it <= ' ' }) is unnecessary
[TrimLambda]
    s.trim({ it &lt;= ' ' }) // HINT 3
           -------------
src/test.kt:6:Hint: The lambda argument ({it<=' '}) is unnecessary
[TrimLambda]
    s.trim() {it&lt;=' '} // HINT 4
             ---------
src/test.kt:7:Hint: The lambda argument ({ c -> c <= ' ' }) is
unnecessary [TrimLambda]
    s.trim() { c -&gt; c &lt;= ' ' } // HINT 5
             -----------------
src/test.kt:8:Hint: The lambda argument ({ it.isWhitespace() }) is
unnecessary [TrimLambda]
    s.trim() { it.isWhitespace() } // HINT 6
             ---------------------
src/test.kt:10:Hint: The lambda argument ({ it <= ' ' }) is unnecessary
[TrimLambda]
    val to = s.trim { it &lt;= ' ' }.substring(2)
                    -------------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the source file referenced above:

`src/test.kt`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers
fun test(s: String) {
    s.trim() // OK
    s.trim() { it <= ' ' } // HINT 1
    s.trim { it <= ' ' } // HINT 2
    s.trim({ it <= ' ' }) // HINT 3
    s.trim() {it<=' '} // HINT 4
    s.trim() { c -> c <= ' ' } // HINT 5
    s.trim() { it.isWhitespace() } // HINT 6
    s.trim() { it <= ' ' || c == '.' } // OK
    val to = s.trim { it <= ' ' }.substring(2)
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/TrimDetectorTest.kt)
for the unit tests for this check to see additional scenarios.

(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("TrimLambda")
  fun method() {
     trim(...)
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("TrimLambda")
  void method() {
     trim(...);
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection TrimLambda
  problematicStatement()
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="TrimLambda" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'TrimLambda'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore TrimLambda ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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